Two experiments investigating the neural basis of olfactory discrimination behavior in rats are proposed: 1. Functions of secondary olfactory pathways in odor detection, detection threshold, intensity difference threshold and 2-odor discrimination will be evaluated after sectioning afferent pathways from the olfactory bulb. Histological confirmation of lesions will include the use of the Fink-Heimer silver stain after bulbectomy in order to determine olfactory projections into the forebrain which were not destroyed by the initial lesions. 2. Acquisition of olfactory learning-sets will be investigated after destruction of olfactory projections to the amygdala and periamygdala cortex and to the mediodorsal nucleus of the thalamus. In the primate, interproblem transfer ability is dependent upon the mediodorsal-frontal cortical system. Because rats acquire learning-set tasks when odor but not when other (visual, auditory) cues are used, this thalamic-cortical system might mediate complex olfactory learning in this species. A wind-tunnel olfactometer and operant conditioning training procedure will be used to assess olfactory sensory threshold and ability to make 2-odor discriminations. The olfactometer allows for precise control of stimulus parameters and the test environment.